Final answer:
Chemistry is the study of the composition, properties, and interactions of matter, including changes in its molecular structure. It involves understanding how substances react and combine and how these reactions can lead to the formation of new substances.
Step-by-step explanation:
The sentence 'A change in the molecular structure or composition of matter' best defines chemistry. Chemistry is the study of the composition, properties, and interactions of matter, including the changes that occur in its molecular structure. It involves understanding how different substances react and combine with each other, and how these reactions can lead to the formation of new substances.